Sliding pressure startup

Yi, T.T., Ishiwatari, Y., Liu, J., Koshizuka, S., Oka, Y., 2005. Thermal and stability considerations of super LWR during sliding pressure startup. Journal of Nuclear Science Technology 42 (6), 537-548. 

The sliding pressure startup systems of the Super LWR and a supercritical FPP are shown in Fig. 1.17 [41]. A steam-water separator is installed on the bypass line for the Super LWR, while it is installed on the main steam line for the supercritical FPP. The Super LWR has an additional heater installed to recover heat from the drain of the steam-water separator. When the enthalpy is low, the drain is dumped into the condenser directly. A boiler circulation pump can be used instead of the additional heater the same as in the sliding pressure FPP. 

Boiling (and dryout) must be prevented in the water rods at subcritical pressures (in sliding pressure startup scheme). 

Fig. 1.17 Sliding pressure startup systems of the Super LWR and supercritical FPP. (a) Super LWR with additional heaters [41] (b) Super LWR with recirculation pumps [41]. (c) Supercritical FPP. (Taken from ref. [41] and used with permission from American Nuclear Society)...

The calculation model for sliding pressure startup of the Super LWR is shown in Fig. 1.18 [43]. Examples of the sliding pressure startup curves based on the thermal considerations are shown in Fig. 1.19 [43]. 

Fig. 1.18 Calculation model for sliding pressure startup scheme. (Taken from ref. [43] and used with permission from Atomic Energy Society of Japan)...

The sizes of the components required for the startup schemes are assessed. The sliding pressure startup with a steam separator in a bypass line is the best from the viewpoint of weight of the components. A study of the times needed for the startup schemes remains as future work. There is a limitation on the rate due to thermal stresses on thick-walled components such as the RPV. In BWRs, the temperature rise rate of the RPV wall is limited to below per hour. 

Fig. 1.22 Revised sliding pressure startup system of the Super LWR and the Super FR...

Fig. 1.23 Redesigned curves of sliding pressure startup before the power raising phase...

Fig. 1.33 Sliding pressure startup curve with thermal and stability considerations...

Dryout or boiling transition may occur during the sliding-pressure startup of the once-through reactor as seen in Fig. 1.21 [43, 44]. Ribbed tubes and spiral tapes have been used for the supercritical boilers to improve the critical heat flux. The... 

There are two kinds of startup schemes currently used in FPPs [1]. One is the constant pressure startup scheme, in which the boiler operates at constant supercritical pressure after the coolant is pressurized to this point. The other is the sliding pressure startup scheme, in which the boiler operates with variable pressures and the pressure increases with the generation output. 

Fig. 5.7 Sliding pressure startup system of Super LWR with recirculation pumps (taken from ref. [2] and used with permission from American Nuclear Society)...

The sliding pressure startup system can be differentiated into six phases described below. 

During subcritical pressure operation in the sliding pressure startup of the Super LWR, a steam-water separator is required to separate the steam and water such that the water can be recirculated to the reactor inlet by recirculation pumps or by additional heaters, in order to maintain adequate core cooling. The size and weight of the steam-water separator are determined by referring to those of sliding pressure supercritical FFPs. The characteristics of the reference 700 MW supercritical boiler and the properties of its steam-water separators are given in Table 5.3. 

Since the Super LWR plant system does not have a superheater, the main steam conditions need to be adjusted during startup and low power operations. The enthalpy of the core outlet coolant must be high enough to provide the required turbine inlet steam enthalpy. At subcritical pressure operation in the sliding pressure startup scheme, boiling and dry out in the descending moderator water rods are undesirable and should be prevented because they affect the inlet subcooling. 

Power Increase Phase in Constant Pressure Startup or Sliding Pressure Startup... 

There is no difference between the pressurization phases of the constant pressure startup and sliding pressure startup schemes because the pressurization phase appears after the line switching to the once-through mode. It is assumed that the core inlet and outlet temperatures are kept equal to their respective values in the normal operating condition. While the reactor core power is increased, the feed-water flow rate is also increased proportionally. MCSTs are calculated for various core powers from 30 to 100% at intervals of 10%, and the calculated results are shown in Fig. 5.11. It is found that MCST satisfies the criterion of 620°C throughout the power increase phase. 

Based on the thermal considerations above, the general startup curves for the constant pressure startup scheme of the Super LWR are designed as shown in Fig. 5.22 [3]. Those for the sliding pressure startup scheme are designed as shown in Fig. 5.23 [3]. 

The startup curve for the sliding pressure startup scheme that is designed based on only the thermal considerations (Fig. 5.23) is redrawn taking the stability considerations into account as well. The constant pressure startup is not discussed here because the partial power operating conditions in the constant pressure startup are covered by the temperature increase phase and power increase phase of the sliding pressure startup. 

Design and Analysis of Procedures for System Pressurization and Line Switching in Sliding Pressure Startup Scheme... 

In the sliding pressure startup scheme, the system pressure of the Super LWR is assumed to be raised by nuclear heating the same as in BWRs. In the thermal and stability considerations for the sliding pressure startup introduced in Sects. 5.3-5.6,... 

From these two background facts, the purposes of this section are to revise the design of the sliding pressure startup scheme, propose its detailed procedures, and assess their feasibility by a system transient analysis. This section covers just the procedures before the power raising phase because the power raising phase itself... 

In this chapter, the plant startup and stability were introduced. Both constant pressure and sliding pressure startup schemes are designed by referring to FPPs. The constant pressure startup system requires a startup bypass system consisting of a flash tank and pressure-reducing valves. The sliding pressure startup scheme... 

Especially for the sliding pressure startup scheme, system pressurization and line switching from recirculation to once-through mode were investigated in detail. The feasibility of the system and procedures for them were assessed by a system transient analysis. 

As described in Chap. 5, the sliding pressure startup is one of the candidate startup schemes. It is necessary to understand the reactor behavior in case of abnormal transients and accidents during the pressurization phase. To do that, SPRAT-DOWN is extended to the SPRAT-DOWN-SUB which can be applied to the transients and accidents during subcritical pressure operation [12]. 

The CR withdrawal at startup is analyzed. The initial condition is the hot standby where kgff is 1.0, the reactor power is 1.0 x 10 of the rated power, and the main coolant flow rate is 20% of the rated flow. The analysis is made assuming adoption of the constant pressure startup scheme described in Chap. 5. The similar analysis for the sliding pressure startup scheme is described in Sect. 6.7.2. The reactivity worth of the withdrawn CR cluster is 2.8%dk/k. The same withdrawal speed as of PWRs is taken. The CR cluster is withdrawn until the reactor period decreases to the scram setpoint (10 s). The inserted reactivity is 0.39. 

---